Many electric linear motion actuators for converting the rotary motion of an electric motor to a linear motion of an object to be driven include a ball-screw mechanism or a ball-ramp mechanism as a motion converter means. Further, many of such linear motion actuators include a gear reduction mechanism such as a planetary gear speed reducer so as to obtain a large linear driving force with a small-capacity electric motor (see e.g. JP Patent Publication 6-327190A).
The ball-screw mechanism or ball-ramp mechanism used in the above-described electric linear motion actuators has the ability to increase the driving force to some extent because it includes threads having a lead angle or inclined cam surfaces. But this motion converting mechanism alone cannot sufficiently increase the driving force as required by e.g. an electric brake system. Thus, the electric linear motion actuators using this type of motion converter mechanism further include a separate speed reducer such as a planetary gear speed reducer to increase the driving force. But the addition of such a separate speed reducer increases the size of the entire electric linear motion actuator.
The inventors of this application proposed an electric linear motion actuator which is free of this problem, which can sufficiently increase the driving force without mounting a separate speed reducer, and which can be used in an electric brake system, of which the linear motion stroke is relatively short. This actuator comprises a rotary shaft to which the rotation of a rotor shaft of an electric motor is configured to be transmitted, a housing having a radially inner surface, an outer race member fitted in the radially inner surface of the housing and provided around the rotary shaft, a carrier, and a plurality of planetary rollers disposed between the outer race member and the rotary shaft and rotatably supported by the carrier, wherein the planetary rollers are configured to revolve around the rotary shaft while rotating about axes of the respective planetary rollers when the rotary shaft rotates, wherein a helical rib is formed on the radially outer surface of the rotary shaft or on the radially inner surface of the outer race member, and wherein each of the planetary rollers has, on a radially outer surface thereof, a plurality of circumferential grooves which are arranged at the same pitch as the helical rib and in which the helical rib is engaged, or a helical groove in which the helical rib is engaged, the helical groove having a different lead angle from the helical rib and being arranged at the same pitch as the helical rib, thereby axially moving the carrier. Thus, the rotation of the rotary shaft is converted to a linear motion of the carrier. In this arrangement, the carrier or a linear motion member coupled to the carrier serves as an output member for linearly driving an object to be driven (JP Patent Publication 2007-32717A and JP Patent Publication 2007-37305A).
On the other hand, while many vehicle brake systems are hydraulic ones, with the recent introduction of sophisticated brake control such as anti-lock brake control system (ABS), electric brake systems are gathering attention because electric brake systems can perform such sophisticated brake control without the need for complicated hydraulic circuits. In an electric brake system, an electric motor is activated in response to a signal produced when the brake pedal is depressed, thereby pressing the braking member against the member to be braked through the above-described electric linear motion actuator, which is mounted in the caliper body (see e.g. JP Patent Publication 2003-343620A).
The electric linear motion actuator disclosed in JP Patent Publication 2007-32717A or JP Patent Publication 2007-37305A, in which the carrier or the linear motion member coupled to the carrier serves as an output member which makes a linear motion, can sufficiently increase the driving force without the need to mount a separate speed reducer and thus is compact in size. But since the carrier or the linear motion member, which undergoes a linear motion, is relatively short in the axial direction, if this electric linear motion actuator is used e.g. in an electric brake system, a tangential force applied from the member to be braked to the braking member as the object to be driven partially acts on the carrier or the linear motion member as a lateral moment. This lateral moment may hamper smooth linear motion of the carrier or the linear motion member. If a linear motion member guided by the outer race member is coupled to the brake member, as the braking member becomes worn, the axial length of the portion of the linear motion protruding from the outer race member increases while the axial length of its portion guided by the outer race member correspondingly decreases. This makes smooth linear motion of the linear motion member even more difficult.
Also, in order to stably transmit rotation of the rotary shaft as the input shaft to the respective planetary rollers, the planetary rollers are disposed between the rotary shaft and the outer race member with negative gaps, thereby applying a preload to the respective planetary rollers between the radially outer surface of the rotary shaft and the radially inner surface of the outer race member. This makes it necessary to mount the planetary rollers between the outer race member and the rotary shaft by e.g. interference fit. Also, in order to control the negative gaps for applying the preload, it is necessary to finish the radially outer surface of the rotary shaft and the radially inner surface of the outer race member with high accuracy by e.g. grinding. It is thus troublesome and time-consuming to finish the rotary shaft and outer race member and then mount the planetary rollers. This increases the manufacturing cost.